Paraxylene is used in the manufacture of polyester which is useful as fibers and polymeric resins. Paraxylene can be manufactured from several feedstocks, the most significant today being reformate produced from naphtha reforming. The two primary processes used to manufacture paraxylene use crystallization or selective adsorption.
Both types of paraxylene manufacturing plants employ heat in the reboiler(s) of a distillation or fractionation tower(s) to separate the tower feed(s) into useful products, including paraxylene. Feed(s) to the tower may come from multiple sources and enter the tower(s) at various locations. The product of fractionation is a stream containing predominantly the xylene isomers including paraxylene that is sent to the crystallization selective adsorption section to recover paraxylene. Crystallization is much less sensitive to impurities in the fractionation product stream than selective adsorption, where these impurities can lead to manufacturing difficulties. Consequently the fractionation employed in crystallization can employ a single tower where the fractionation product stream can be removed as a side stream.
In both processes, the heat needed to drive fractionation is typically supplied in the reboiler, either in a fired heater or via heat exchange with steam, and removed in a condenser with the tower overhead product, typically via air and/or cooling water-cooling. If the fractionation tower operates at close to ambient conditions, the heat removed in the overhead condenser is often too low in temperature or thermodynamic quality to be useful. Both types of paraxylene manufacturing processes can raise the pressure of the fractionation tower so that the temperature or thermodynamic quality of the heat removed in the overhead condenser is higher and useful in other parts of the paraxylene manufacturing plant. However raising the tower pressure increases the cost of the fractionation equipment and lowers the separation efficiency. Thus, there is a need to recover heat in the fractionation employed in paraxylene manufacturing without raising tower pressure to increase the temperature or thermodynamic quality of the heat.
Side condensers located below the overhead condenser on the fractionation tower(s) can remove the heat at higher temperature. It is important to note that the feed to the fractionation tower(s) of paraxylene manufacturing plants often contain low molecular weight gases. One disadvantage of side condensers is that the heat removed by condensing the tower side condenser product contains low molecular weight gases. There are problems associated with the flashing of these low molecular weight gases if side condensers are employed. Thus, there is a need for a process and apparatus to effectively recover heat free of low molecular weight gases in a paraxylene manufacturing process.